Over the past thirty years, significant attention has been given to the production of ethyl alcohol, or “ethanol,” for use as an alternative fuel. Ethanol not only burns cleaner than fossil fuels, but also can be produced using corn, a renewable resource. At present, more than sixty-nine “dry milling” plants in the United States produce billions of gallons of ethanol per year. Additional plants presently under construction are expected to add hundreds of millions gallons to this total in an effort to meet the current high demand.
As noted in the foregoing discussion, a popular method of producing ethanol from corn is known as “dry milling.” As is well known in the industry, the dry milling process utilizes the starch in the corn to produce the ethanol through fermentation, and creates a waste stream comprised of byproducts termed “whole stillage” (which may be further separated into products commonly referred to as “distillers wet grains” and “thin stillage”). Despite containing valuable oil, these byproducts have for the most part been treated as waste and used primarily to supplement animal feed. This feed is mostly distributed in the form of distillers dried grains with solubles, which is created by evaporating the thin stillage, recombining the resulting concentrate or syrup with the distillers wet grains, and drying the product to have a moisture content of less than about 10% by weight.
Efforts to recover the valuable oil from these byproducts have not been successful in terms of efficiency or economy. For example, one approach involves attempting to separate the oil from the thin stillage before the evaporation stage, such as using a centrifuge. However, spinning the thin stillage at this stage does not produce usable oil, but rather merely creates an undesirable emulsion phase requiring further processing. Moreover, the volume of thin stillage is generally 2 to 10 times greater than the syrup, and thus involves a considerable capital requirement to purchase the number of centrifuges required. Together, these obstacles make attempts to recover oil from corn thin stillage prior to evaporation highly inefficient and uneconomical.
U.S. Pat. No. 5,250,182 (the disclosure of which is incorporated herein by reference) describes the use of filters for removing substantially all solids and recovering lactic acid and glycerol from the thin stillage without the need for evaporation. Despite eliminating a step in the conventional process, the proposal results in a more complicated arrangement requiring multiple filtration steps. Wholesale elimination of the evaporator in the vast majority of existing plants is also unlikely and otherwise uneconomical. Filters, and especially the microfiltration and ultrafiltration types proposed for use in this patent, are also susceptible to frequent plugging and thus deleteriously increase the operating cost. For these reasons, the filtration process proposed in this patent has not gained widespread commercial acceptance.
Accordingly, a need exists for more efficient and economical manners of recovering oil from byproducts created during the production of ethanol.